Coherent optical communication systems have been developed that provide distinct advantages over more traditional direct detection schemes. In a coherent optical communication receiver, an optical signal is transmitted to a receiver, which converts the optical signal into corresponding electrical signals. The optical phase and amplitude of the transmitted optical signal are then detected based on the electrical signals, thus enabling the use of multilevel modulation schemes to increase optical fiber spectral efficiency. Coherent detection provides another advantage over direct detection in that linear impairments of the transmitted optical signal can be compensated for in the receiver using digital filters and other known digital compensation techniques that operate on the electrical signals associated with the received optical signals. Demodulation of the received optical signal is commonly performed using digital signal processing techniques in the receiver. In known systems these digital signal processing functions rely on clock phase/timing recovery of the received signal and synchronization of the receiver clock in order to accurately demodulate the received signal. Reliable and efficient clock phase recovery, however, is adversely affected by the degree of impairment and distortion of the received signal. Accordingly, in some known systems clock recovery cannot be performed until the impairments are compensated for in the receiver, which can be very difficult for large amounts of distortion resulting in unnecessary delay in initializing the receiver.
Some significant linear impairments and signal-distorting phenomena that can affect the performance of optical communication systems include chromatic dispersion (CD) and polarization mode dispersion (PMD). The transmitted optical signal has a finite spectral width such that the spectral components may be transmitted at different frequencies. Chromatic dispersion is a result of different spectral components of an optical signal propagating at different speeds along an optical fiber, resulting in an undesirable temporal spreading of the optical signal. PMD occurs due to the different polarization modes (X and Y polarization components) of the optical signal propagating along the fiber at different speeds and is caused by environmental effects and asymmetries in the optical fiber, which are often random, unpredictable and can vary over time. A differential group delay (DGD) describes the delay between the X and Y polarized signals as a result of the PMD effects in the optical fiber. Due to the random and unpredictable nature of PMD, PMD often cannot be easily determined or compensated for in a conventional optical communication system.
Optical communication systems generally include a number of links of optical fibers and optical system components, each contributing to the overall chromatic dispersion of the received signals. One conventional method of compensating for chromatic dispersion in an optical communication system includes the use of dispersion compensation fibers (DCFs) or other components that compensate for chromatic dispersion optically. In practice, however, implementing such DCFs and other known dispersion compensating techniques and components in the optical communication system undesirably increases the size and complexity of the system, and can be cumbersome and expensive. Additionally, such dispersion compensation components can undesirably limit the power and spectral efficiency of the optical communication system.
Recent advances in coherent receiver technology allow for compensation of linear transmission impairments, such as chromatic dispersion, by utilizing electronic dispersion compensation in the receiver instead of performing optical compensation using DCFs, such as those described above. Such electronic dispersion compensation operates on electrical signals generated in response to a received optical signal. One challenge related to these techniques, however, is that without the use of DCFs, a coherent optical receiver should be designed to compensate for a large chromatic dispersion, such as values as high as 51,000 ps/nm or higher. Another challenge is that since the received signal is affected by both chromatic dispersion and PMD, it is more difficult to singularly estimate the effects of either one. In conventional systems, discussed above, the chromatic dispersion is commonly compensated for first, or otherwise considered to be negligible, so that the PMD can be more accurately estimated at the receiver. In a system with potentially large chromatic dispersion and PMD, it is very difficult to accurately and efficiently estimate these effects in the receiver, thus seriously degrading performance of the optical communication system.
Some known methods for dealing with chromatic dispersion at the receiver utilize a preset or adaptive filter to compensate for a known chromatic dispersion. These solutions, however, are limited in that in many systems, especially switched systems in which the signal may travel via one of several different links, the chromatic dispersion effects cannot be easily known or determined in advance because each link scenario has a different chromatic dispersion, or the adaptive filter cannot be easily or efficiently updated.
In other known systems the chromatic dispersion may be estimated by iteratively scanning a range of chromatic dispersion values for the received signal over a number of steps and testing for each step whether satisfactory sensitivity is achieved in a control loop, such as an automatic gain control (AGC) loop or clock recovery loop. The scanning is performed by estimating a first value for the chromatic dispersion (such as 1000 ps/nm), calculating a chromatic dispersion coefficient, using the calculated chromatic dispersion coefficients in a compensation filter and evaluating the control loop response, then revising the estimate based on the control loop response. The range of the estimated chromatic dispersion values should be relatively narrow to minimize estimation error and ensure that the control loops achieve a desired sensitivity, that is, to ensure a clock recovery loop achieves a lock point such that the loop is stable. For example, the estimated chromatic dispersion value in each step should differ by 1000 ps/nm, or less, per step. To compensate for a chromatic dispersion of up to 50,000 ps/nm (from the above example), the scan may require approximately 50 steps or iterations. For each iteration, the step of calculating the chromatic dispersion coefficients results in even greater complexity. Additionally, there is added delay in such systems due to the time required for the acquired signal to be allocated for the chromatic dispersion scanning steps discussed above.
Additionally, the chromatic dispersion estimation methods discussed above fail to discriminate between actual chromatic dispersion effects and the effects of the second and higher order components of PMD present in the optical communication system. Second order PMD is characterized as the derivative of the first order PMD with respect to frequency. Because second order PMD is a function of frequency, it mimics chromatic dispersion. If the estimated chromatic dispersion in these known methods includes the effects of second order PMD, the receiver sensitivity can be seriously corrupted.
Another disadvantage of such scanning methods is that the chromatic dispersion cannot be accurately estimated until the clock recovery loop is locked. The above scanning methods typically only partially compensate for the chromatic dispersion in order to recover the timing signal. Any residual chromatic dispersion is then compensated for once the receiver clock is synchronized. These methods, however, are inefficient and unreliable since the clock recovery loop cannot lock with a satisfactory sensitivity until the signal impairments are fully compensated. This requires a high degree of complexity in the receiver and results in considerable increase in processing time and unsatisfactory delay in initializing the receiver.
Furthermore, for optical communication systems with large PMD resulting in a high DGD such as one half symbol period (i.e., T/2, where T is the symbol period), the above receiver techniques do not enable satisfactory sensitivity of clock timing recovery functionality with respect to the PMD effects of the received signal, thus resulting in poor system performance. Reliable clock timing recovery enables a receiver clock to “synchronize” with a clock phase of the received signal in order to properly demodulate the received signal. For certain polarization conditions or states of the received signal, a clock phase cannot be detected on either of the received X-polarization signal, the Y-polarization signal, or a combination of both. For example, FIG. 6 illustrates a simplified first order PMD model of fiber channel 12 in an illustrative embodiment. Block 10 represents a transmitted optical signal of X-polarization (xpol) and Y-polarization (ypol). The PMD model comprises a first order PMD block 20 which rotates the transmitted signals by a value J1=θ1, a DGD block 30, representing a delay of T/2, and PMD block 22 which rotates the signals by a value J2=θ2. In the fiber channel 12, with θ2=45° in the PMD model, the clock phase detector 40 will detect two random (Xpol and Ypol), uncorrelated signals at T/2 delay with respect to each other. Accordingly, the clock phase detection on either of the X-polarization signal or the Y-polarization signal of the two uncorrelated signals will cancel each other out and result in a modem failure in the receiver.
Some known systems have addressed the PMD problem. The known techniques, however, lead to clock recovery loop lock points of the signal that are a function of the detected PMD conditions in the fiber channel. The lock points indicate a clock phase used in the sampling of the received signal, as discussed further below. Therefore, the receiver clock phase varies widely in response to changes in the PMD conditions of the fiber channel, which are not static and can vary significantly over time and with changes in temperature of the channel. Further, the time-varying nature of the PMD conditions result in clock jitter that can accumulate over multiple regeneration nodes in known systems and cause adverse affects in the clock phase detection circuitry.
There is a need, therefore, for an efficient, yet robust method of detecting a clock phase of the received signal for synchronizing the receiver clock with a transmit clock that is not affected by the PMD effects in a fiber channel. Thus, it is desirable to implement a method of detecting a clock phase that is tolerant to the PMD effects on the received signal even when a DGD is determined to be up to or in excess of one-half the symbol rate, or baud rate. Particularly, it is desirable to efficiently and reliably detect the clock phase of a received signal prior to and/or independent from any compensation of the effects of chromatic dispersion and PMD on the received signal.